This project deals with a biochemical and genetic analysis of bacterial mutants which are unable to assemble ribosome subunits at low temperature. Among such mutants we have found several groups whose genetic lesions map at various sites on the E. coli chromosome. Because none of the present mutants map in regions known to contain the genes for ribosomal proteins and ribosomal RNA, they are assumed to be altered in factors that participate in the ribosome assembly and maturation process, hence, are called rim for ribosome maturation. One such mutant, rim A, accumulates 43S ribosomal particles which apparently are complete with respect to their molecular constituents, but yet which differ from mature ribosones in their sedimentation coefficient and their lack of activity in an in vitro protein synthesizing system. We will pursue our studies on the rim A mutant, and a factor isolated from wild type cells which promotes the conversion of 43S precursor particles to "50S" ribosomes. The factor which promotes this conversion will be purified and its properties in the conversion will be studied in detail. We will also investigate the possibility that the factor promoted conversion of precursor particles involves guanosine tetraphosphate (or some other guanine nucleotide). Other mutants in the collection which have assembly properties similar to the rim A mutant will be studied in detail with the aim of isolating those factors from wild type extracts which will complement the apparent mutational defect in vitro.